Paper containers, publications, cards, posters and other printed paper products include many of those having a plastic film laminated to the printed surface to protect the surface, render the surface resistant to water and oils or beautiful and for other purposes as well known. The article thus treated is generally termed a "print laminate" in the art. It is common practice to use a composite film comprising a film substrate and a heat-sensitive adhesive resin layer as a film for print laminates. Usually, the heat-sensitive adhesive resin layer of the film is bonded to a print such as printed paper by thermocompression to prepare a print laminate, i.e. the print covered with the film laminated thereto. The prior art concerned is disclosed in Unexamined Japanese Patent Publication No. SHO 59-24666, Examined Japanese Patent Publication No. SHO 60-36939, Unexamined Japanese Patent Publications Nos. SHO 60-135236 and SHO 60-135237, etc.
However, the print laminate products thus obtained have a drawback in that when a plastic film is laminated to printed paper or like print, the print color generally appears darker. Presumably, this is attributable to the following reason. The color printed on paper or the like before lamination appears slightly more whitish than the original color of the printing ink owing to the diffused reflection of light at the surface of the print, whereas no diffused reflection occurs when the plastic film is laminated to the print since the heat-sensitive adhesive resin layer bonds to the surface of the print intimately completely.
On the other hand, the printer is responsible for finishing the print in the color requested by the user. However, the color, even if printed as specified, appears different upon lamination, with the result that he fails to assume his responsibility. Accordingly, it has been strongly desired to prevent the color change of print laminate products due to lamination.
In view of the above problem, we have conducted extensive research and found that when a finely divided white material is provided at the interface between printed paper or like print and a film laminated thereto, diffused reflection occurs owing to spaces around fine particles, consequently preventing the color from appearing darker upon lamination to preclude the color change.
The object of the present invention, which has been accomplished based on the above finding, is to provide a film for print laminates and a thermocompression bonding process therefor which permit printed paper or like print to retain its original color free of any change despite lamination.